1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser beam printer for doing printing by scanning light of a laser beam and an electrophotographic process, and particularly to a two-dimensional beam writing position detecting device used therein.
2. Description of the Related Art
A conventional laser beam printer performs exposure of a semiconductor laser in binary of ON and OFF, and forms an electrostatic latent image of an image on a photoconductive drum. At this time, a beam detector is provided in a position on beam scanning and other than on a scanning line of a photoconductor in order to determine a writing position on the photoconductor. A high-speed type PIN photodiode is used in this beam detector since a fast response is highly required.
In recent years, a speedup in a printing speed of a printer advances and a semiconductor laser becomes multiple beams and a method of scanning by plural lines is considered, and when a light source of the semiconductor laser is arranged, for example, in two dimensions with three by three (nine), plural beams are scanned in a beam detector.
Hence, when a slit is provided in the front of the beam detector and scanning is performed at a distance between each the beam sufficiently wider than a width of the slit, detection can be performed, but due to restrictions on an optical system and the distance between the beams for scanning a photoconductor and so on, as shown in FIG. 1 described below, it becomes an arrangement so that the beams of the second and subsequent rows overlap with the beam of the first row with respect to a sub-scanning direction, and at least two or more beams overlap on a photodetector of the beam detector in the sub-scanning direction even when the slit is provided.
Generally, by converting an optical signal detected by the photodetector into a voltage signal and comparing a rise portion of this voltage with a reference voltage, a signal for determining a writing position on the photoconductor is obtained, but as described above, the plural beams overlap within the slit at a time difference, so that each the beam cannot be split.
A waveform in the case of making a voltage conversion of an output from the beam detector when scanning is performed at a slight distance (distance by a radius of the beam) of the sub-scanning direction is shown in FIG. 8. As shown in the drawing, since distortion (stepwise waveform) occurs in a rise voltage of an optical signal, a detection signal of a beam B1 is obtained at a level of a reference voltage 1 (reference level 1), but light of beams B1, B4, B7 overlaps at levels of reference voltages 2, 3 (reference levels 2, 3) and positions of the original beams B4, B7 (original detection position) cannot be detected, so that problems that a writing position of the beam cannot becomes unstable etc. occurred.
An object of the invention is to provide a two-dimensional beam writing position detecting device capable of eliminating a disadvantage of such a conventional art and splitting writing signals of each beam of a two-dimensional laser diode for performing multiple beam scanning by one beam detector and determining a writing position on a photoconductor, and an image forming apparatus using the detecting device.
In order to achieve the object, first means is a two-dimensional beam writing position detecting device for providing an optical system for scanning on a photoconductor by laser beams emitted from a semiconductor laser to form an electrostatic latent image and arranging a plurality of the laser beams in two dimensions and slantingly scanning each the laser beam for forming the electrostatic latent image on the photoconductor at a predetermined angle (xcex8) and detecting the laser beams for determining the first writing position on the photoconductor of the laser beams, characterized in that a longitudinal direction of a beam light receiving surface of the detecting device inclines at the substantially same angle (xcex81) as the slant scanning angle (xcex8) with respect to the perpendicular of a scanning direction of the plural beams.
Second means of the present invention is characterized in that in the first means, the angle (xcex81) of inclination of the longitudinal direction in the beam light receiving surface of the detecting device is within the range represented by the following expression.
xcex81=xcex8xc2x1tanxe2x88x921[a beam radius/(P2xc3x97the number of beams of a primary scanning direction)]
where P2 in the expression is a beam pitch of a sub-scanning direction.
Third means of the present invention is characterized in that in the first or second means, a length S1 of a laser beam sub-scanning direction of the beam light receiving surface of the detecting device is more than or equal to a value in which a beam diameter is added to a value in which a beam pitch P2 of the sub-scanning direction is multiplied by [(the number of beams of the sub-scanning direction)xe2x88x921], and a length S2 of a laser beam scanning direction of the beam light receiving surface is less than [(a beam pitch P1 of the scanning direction)xe2x88x92(a beam diameter)].
Fourth means of the present invention is characterized in that in any of the first to third means, the beam light receiving surface of the detecting device is partitioned and formed by a slit.
Fifth means of the present invention is characterized in that in any of the first to third means, the beam light receiving surface of the detecting device is formed by a photodetector.
Sixth means of the present invention is characterized in that a signal detected by a scanning direction beam of the first row or the plurality-th row on a beam light receiving surface of the detecting device is formed into a writing position signal on a photoconductor of the scanning direction beam of the first row, and a writing position signal on the photoconductor of the scanning direction beam of the second or subsequent row is formed into a signal in which a particular delay or lead is provided so that a scanning direction writing position on the photoconductor matches with the beam of the first row with respect to the signal obtained by the above.
Seventh means of the present invention is a two-dimensional beam writing position detecting device for providing an optical system for scanning on a photoconductor by laser beams emitted from a semiconductor laser to form an electrostatic latent image and arranging a plurality of the semiconductor laser beams in two dimensions and slantingly scanning each the laser beam for forming the electrostatic latent image on the photoconductor at a predetermined angle (xcex8) and detecting the laser beams for determining the first writing position on the photoconductor of the laser beams, and is characterized in that a signal detected by a scanning direction beam of the first row or the plurality-th row on a beam light receiving surface of the detecting device is formed into a writing position signal on the photoconductor of the scanning direction beam of the first row, and a writing position signal on the photoconductor of the scanning direction beam of the second or subsequent row is formed into a signal in which a particular delay or lead is provided so that a scanning direction writing position on the photoconductor matches with the beam of the first row with respect to the signal obtained by the above.
Eighth means of the present invention is characterized in that a two-dimensional beam writing position detecting device is installed in a position on beam scanning and other than on a scanning line of the photoconductor.